1. Field of the Invention
The present invention generally relates to electronic devices designed to enhance the safe operation of railway vehicles by monitoring the alertness of the engineman, i.e., so-called "dead man" devices, and more particularly to an improved timer circuit for such devices which allows a "drag operation", such as occurs when gondola cars are slowly dragged past a loading station, without frequent operation of the audio/visual alarms.
2. Description of the Prior Art
Electronic devices are known which monitor in an unobtrusive manner the actions taken by an engineman in controlling a locomotive of a train. After a predetermined period in the absence of detectable control movements, the system requests an acknowledgment from the engineman by means of audio/visual alarms. Failure to acknowledge results in a power-down sequence which ultimately brings the vehicle to a safe and complete stop.
One such product is sold by Pulse Electronics, Inc., of Rockville, Md., under the trademark of "Train Sentry II" as described in Bulletin 10781-B entitled "Train Sentry II Operating Instructions", revised May 28, 1986. The "Train Sentry II" is composed of a light/horn box located near the control stand in the locomotive cab and a control box mounted elsewhere. Each subsystem contains a timer and a distance counter, both of which are programmable and either one can trigger the warning and stop sequence. The timer stores the time elapsed since the last reset. The distance counter stores a distance since last reset. The timer controls at lower speeds, while at higher speeds, the distance counter controls. If the time interval or the distance reaches predetermined programmed values, the system assumes that the engineman is disabled and asks him, by means of rapidly pulsating lights and warning horns, to acknowledge. The intermittent action of the warning devices gradually increases in intensity and if, at the end of the warning period the engineman has not responded, the equipment concludes that he is disabled and de-energizes a normally closed magnetic air valve connected to the air brake system. This action establishes a well controlled power-down sequence that results in stopping the train.
The time required for the system to request an acknowledgement is factory programmed in accordance to customer specifications. FIG. 1 presents a specific example of the timing functions. The time sequence is described by the following examples with reference to FIG. 1. Between 0 and 3 mph, the reset window is 20 seconds. This is to prevent tampering with the device such as cutting the wires from the speed sensor. For train speeds between 3 and 20 mph, the reset window is 120 seconds. Above 20 mph, the reset window is computed as a function of speed, and in the specific example illustrated, it is 2400 divided by the speed in mph. It will, of course, be understood that the values given in this example are merely illustrative and, in fact, are programmable. Resets to the equipment are generated as a result of monitored actions of the engineman, such as operating the throttle, brakes, horn and the like. Every time a reset is received by the equipment, the timers are reset to zero thereby waving the need for the engineman to acknowledge. When an acknowledgement is required by the engineman, the acknowledgement acts as a manual reset.
The reset window timing illustrated in FIG. 1 is optimal for most railroad operations. There is, however, a special case where the window timing has proved to be an irritant to enginemen and that is the case of a so-called "drag" operation of the type where a long train of gondola cars is being loaded with, for example, coal. In this operation, the gondola cars are slowly "dragged" past a loading station at a speed such that each car will be fully loaded by the time it passes the loading station. During this operation, the speed of the train is very slow, normally about one mph, and there are minimal functions the engineman must perform. As a result, the times between monitored actions can be quite long unnecessarily activating the warning lights and horns and requiring the engineman to acknowledge. The problem is made all the worse by the 20 second reset window for speeds between 0 and 3 mph.
A straight forward solution to this problem might simply be to provide the engineman monitoring device with either a disable switch or a switch to increase the time period of the reset window. However, neither of these solutions are acceptable for reasons of safety. These devices must always be fully operational no matter what operation is being performed; therefore, a disable switch is not a good solution. Moreover, a switch to increase the time period of the reset window could be a safety hazard if the engineman forgets to, or intentionally does not, reset the switch to the regular reset window operation. Further, it has been observed that some enginemen do not like the monitoring devices and will sometimes resort to extreme measures to disable the devices.